1. Field of the Invention
The present invention relates to an image forming apparatus that uses an electrophotographic method or an electrostatic recording method. More particularly, the invention relates to an image forming apparatus, such as a copying machine, a printer, and a FAX, among some others.
2. Related Background Art
Generally, a monocomponent developer using magnetic toner as the main components or two-component developer using non-magnetic toner and magnetic carrier as the main component thereof are used for the developing device that serves as developing means provided for an image forming apparatus using an electrophotographic method or an electrostatic recording means. For the image forming apparatus that forms full color or multicolor images by means of the electrophotographic method, in particular, most of the developing devices thereof adopt the two-component developer in consideration of color tones of images or the like.
As is well known, the toner density (the ratio of toner weight to the total weight of carrier and toner) of the two-component developer is an extremely important factor for stabilizing the image quality. During development, the toner of a developer is consumed to reduce the toner density of the developer. Therefore, the toner density in a developer or image density should be detected timely in order to replenish toner depending on such changes. Then, the toner density or image density is controlled to be constant at all times for the maintenance of image quality.
Here, conventionally, density control devices of various types have been proposed for the constant control of the toner density or image density by performing the toner replenishment control for a developing device the toner density of which has been reduced.
For example, as a method for keeping the toner density constant in a developer in a developing container of a developing device, there is the one that controls the toner density of the developer in the developing container by detecting the amount of reflected light therefrom (developer reflection control) or the one that makes control by detecting the magnetic permeability of the magnetic carrier in a developer in a developing container (inductance control), among some others.
However, the method for keeping the toner density of a developer constant is arranged to detect the toner density of the developer directly. Therefore, although it is possible to keep the toner density in the developer constant, the developing capability is caused to change if the amount of friction charging (triboelectrification) of toner changes due to a long term use that allows the quality of toner or magnetic carrier in the developer to change, leading to a drawback that the image density is also caused to change concurrently.
For recent image forming apparatuses, therefore, it is generally practiced to adopt a method (patch detection control) for controlling the developer density in which a reference image (hereinafter referred to as a “patch image”) is formed on a photosensitive drum serving as image bearer, and then, the amount of toner adhesion thereto is detected by an optical detection sensor serving as density detection means arranged to face the photosensitive drum. With this patch detection control method, it becomes possible to keep the image density substantially constant, because the amount of toner adhesion of the patch image on the photosensitive drum is detected to effectuate the replenish control. However, there has been known a great influence that may be exerted by the sensitivity characteristics of the photosensitive drum and the like.
Here, the specific description will be made in conjunction with FIG. 3. FIG. 3 shows the E-V characteristics divided by 8 bits, in which the axis of ordinate indicates the potential (V) on a drum, and the axis of abscissa indicates the laser level. In FIG. 3, the sensitivities of the photosensitive drum before and after duration are indicated. It is understandable from the representation of FIG. 3 that the halftone sensitivity of the photosensitive drum used for this examination is deteriorated after duration. Also, the density of the patch image which is used for the patch detection control is a halftone density with the characteristics of the optical detection sensor and the image stability taken into consideration.
As a result, if the patch detection control is made on a photosensitive drum after duration where the sensitivity has been deteriorated, the contrast potential of a patch image is reduced, because the halftone latent image potential, that is, the potential of the patch latent image, is higher than the one initially set. Consequently, the developing characteristics of the patch image is made lower than the one before duration. Then, in order to compensate for this reduction of developing characteristics, control means should repeat toner replenishment. Thus, changes take place as indicated by developing characteristic curves indicated in FIG. 4 when the developing characteristics are restored to the initial ones. In this respect, the image density shown in FIG. 4, that is, the patch image density, is measured at 0.8 of the X-rite reflecting densitometer.
Now, as described earlier, the quality of toner or magnetic carrier in a developer should change due to a long term use, and if the recovery thereof becomes difficult only by means of toner replenishment control, the developer should be replaced then or before such difficulty may take place. Thus, the initial status of the developer should be detected to set the reference value of patch image density (hereinafter referred to as the “initial setting”) when the developer is replaced (which includes the replacement of toner replenishment container that contains toner to be replenished to a developing device detachably mountable on an image forming apparatus). This initial setting is an extremely important sequence, because it provides the target value of riding amount of toner on the photosensitive drum thereafter. However, if the initial setting is executed in a status that the sensitivity of photosensitive drum is allowed to vary due to the individual difference or durability deterioration between each of the photosensitive drums, it inevitably exerts great influence on the image density transition after the initial setting.
Also, at the time of usual initial setting, the patch laser levels, which are determined by the installation environment or the environmental history of an apparatus, are indicated on a table or the like, and used after having been stored on the main body of the apparatus. However, the aforesaid patch levels are fixed and used for the initial setting even after developer has been replaced. As a result, the initial setting is made inevitably in a status where no compensation is provided for the latent potential of the patch image.
As described above, with the conventional patch detection control at the time of initial setting which is needed after developer replacement or the like, the developing characteristic curve after duration is caused to change under any circumstances due to the inadequate setting for the initialization which is accompanied by the changes of photosensitivity characteristics of the photosensitive drum. Then, it is often encountered that the resultant image density changes greatly.